This invention relates to a brake pressure control valve for use in a vehicle hydraulic braking system.
Conventionally, a hydraulic pressure control valve is provided in an automotive hydraulic braking system for controlling the braking pressure in the rear wheel brake cylinders relative to that of the front wheel brake cylinders such that the pressure in each rear wheel cylinder (RWC) is equal to that in the front wheel cylinders (FWC) when the pressure is below a predetermined level and such that when the pressure exceeds the predetermined level the pressure in RWC increases at a reduced rate as compared with the pressure in FWC. The predetermined level is usually referred to as the split pressure. Such control valves usually comprise a piston having a valve seat or a valve member, and a spring urging the piston in the valve opening direction. When the pressure acting on the piston exceeds the split pressure, the piston displaces against the force of the spring thereby closing the valve.
Japanese Patent Disclosure (Kokai) No. 54-155531 shows a brake pressure control valve of the kind described, wherein a pair of valve units are arranged side by side in a housing, and the valve units control the pressure in separate pressure circuits. A single spring acts commonly on respective pistons of the valve units through a common spring seat. The construction is compact and simple and the operation of the valve is generally satisfactory. However, there are shortcomings such as that the frictional resistance in the respective pistons may not necessarily be equal to each other and the locations at which the common spring seat contacts the respective pistons may differ from each other, whereby it is difficult to equalize the split pressure in the respective pressure circuit. In hydraulic braking system having separate pressure circuits. In left and right rear wheel cylinders are usually connected to respective pressure circuits and, therefore, in the foregoing case the braking force in the left and right rear wheels can not be balanced.
The brake pressure control valve disclosed in the Japanese Patent Disclosure further provides a countermeasure for compensating for the failure in either of the pressure circuits, wherein the spring seat is guided on a tapered guide rod for allowing the inclination of the spring seat, and a stop is provided on the guide rod for restricting the relative axial displacement of the spring seat when the spring seat inclines with respect to the guide rod upon the failure in one pressure circuit.
It will be noted that there are many proposals for brake pressure control valves wherein two pistons are arranged in line; however, the overall length thereof is relatively great, and there is a problem in installation.